Add defcustom markdown-wiki-link-retain-case

[shipmints]

Description

This option to nil to retain existing behavior. When set to t, it prevents markdown-convert-wiki-link-to-filename from altering the case of the user's input.

Related Issue

Satisfies #839.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the CONTRIBUTING.md document.
• I have updated the documentation in the README.md file if necessary.

Looks like this hasn't been happening recently. Happy to.

• I have added an entry to CHANGES.md.

Ditto.

• I have added tests to cover my changes.
• All new and existing tests passed (using make test).

Should add this option:

• byte compiler run with no warnings or errors.